Why don't humans have tortoiseshell hair?
Hi Chris, Looking around at the neighbourhood cats, it struck me that most of them (and indeed most mammals) have patterned fur with several different colours.
This made me wonder why Humans tend to have uniform hair colour on their heads, faces & bodies. Why is this? Has there ever been anyone with multicoloured or patterned hair, or is it always a uniform colour? Regards, James in Cambridge.
Ian - I'm Professor Ian Jackson at the MRC Human Genetics Unit in the University of Edinburgh. We understand a bit about the genetics of human colour. We know the gene that causes red hair for example and we know some of genes make long versus dark hair. But in humans, we never see coloured patterns in their hair.
Now in cats, and many other mammals, we see a whole range of patterns. Cats have this stripy tabby pattern. So the stripes are caused by a gene called amino peptidase q and when this gene is missing, the cats lose that stripy pattern and have a more blotched pattern where the stripes become irregular. We humans have this amino peptidase q gene, it really isn't going to do the same thing in humans. We don't actually know what it's doing.
Cats also have that tortoiseshell or calico pattern. That's caused by a gene on the X chromosome that makes orange pigment and females have 2 X chromosomes. Females shut down one of the X chromosomes in random in different cells. So, if she has 1 orange X chromosome and 1 black X chromosome, then you get a mixture of orange and black patches. It's the classic tortoiseshell pattern and tortoiseshell cats are always female.
Hannah - Also, the pointed pattern in Siamese cats where the ears, nose and extremities are darker stems from a mutated colour gene that switches off at higher body temperatures.
So, humans seem to have simple genetic rules for hair colour and they don't respond to the patterning genes in the same way as cats, which is why you don't generally get a tabby human.
There are exceptions to this though. People with piebaldism do show patches of white hair even at a young age. Their melanocytes, so the cells that produce the pigment for hair colour have been mutated and switched off, and it's this that produces the colourless patches.